The prior art is replete with different types of current source circuits for providing an output current that is a function of some parameter (transistor scaling, resistance, temperature, etc.) of the circuit. For instance, a basic current source that is well known in the art is the simple current mirror circuit comprising a diode-connected transistor having its emitter and base coupled in parallel with the emitter and base of a second transistor. The commonly connected emitters of each transistor are connected to a source of operating potential with the collector of the diode-connected transistor being coupled to a source of input current. The input current is mirrored through the second transistor wherein the current flowing in the collector thereof is equal in magnitude to the value of the input current. Moreover, as is understood, the value of the collector current flowing in the second transistor can be made any ratio of the input current by area ratioing the emitter areas of the two transistors. The disadvantage of this circuit is that errors are inherent therein which prevents the absolute matching of the output current to the input current. The most significant cause of this error is the base current error associated with the two transistors, especially if these devices are PNP transistors. Additionally, this circuit is subject to errors induced by power supply ripple variations due to finite output impedance.
Other bipolar prior art current sources reduce the errors associated with the PNP base current flow but suffer in that higher voltage drops across the current source are required in order to provide a current at the output.
A known bipolar switchable precision current source, U.S. Pat. No. 4,423,357, comprises a first transistor coupled between a power supply and a load. Two other complementary transistors have their emitters coupled together and their collectors coupled between the base and collector of the first transistor. One of the two complementary transistors has its base coupled to a switching signal source for switching the current on and off and the other has its base coupled to a reference voltage for regulating the current through the first transistor.
Many MOS transistor constant current sources also exist in the prior art. Typically, first and second transistors are coupled in series and are coupled in parallel with third and fourth transistors that are coupled in series. One of the transistors in at least one of the series connected transistors is coupled as a diode. An output transistor has a gate coupled to a node between one of the series coupled pair. However, these do not provide the pulsating output from a switchable input.
Thus, what is needed is a low voltage, switchable MOS transistor current source.